1. Field of the Invention
The present invention relates to planar antennas, and more particularly, to a planar broadband dipole antenna capable of linearly receiving and transmitting waves over a wide band.
2. Description of the Related Art
Various planer antennas are depicted by: U.S. Pat. No. 4,318,109 to Paul Weathers entitled Planar Antenna With Tightly Wound Folded Sections which describes a broad-band antenna system capable of receiving VHF, FM, and UHF bands, providing sharp nulls for the rejection of unwanted reflections, and having broad directional properties and no radiation capabilities. Cited as a background reference of a planar broad-band antenna; U.S. Pat. No. 5,563,616 to Richard C. Dempsey, et al. entitled Antenna Design Using A High Index, Low Loss Material which describes an antenna having a dipole element which includes two bow-tie shaped arms positioned on a high index of refraction substrate, the opposite surface of which is covered by ground plane. Signal power is applied to (or received from) the arms by balanced feed lines. The construction of dipole element is similar to that of a conventional dipole element in that it is formed by depositing, plating or etching the metal arms on the substrate; U.S. Pat. No. 5,748,152 to John R. Glabe, et al. entitled Broad Band Parallel Plate Antenna which describes a broad-band antenna formed from a relatively thin metal layer (e.g., copper) deposited on a major surface of an electrically insulative substrate. The metal layer has been etched away to leave first and second slot sections of identical symmetrical shape, the two symmetrical slot sections serve as the two antenna elements that form the slot antenna. A top metal plate, sheet or layer of copper or other conductive material is disposed above the antenna so as to be closely spaced and parallel or nearly parallel to the antenna. The metal plate having the back edge and a forward edge which is relatively transverse to an axis defined by the transition portion. To prevent radiation leakage out the back, the back edge of the metal plate is shorted or grounded to the antenna by means of a back or rear metal plate of copper or other conductive material which is nearly perpendicular or orthogonal to the metal plate and the antenna. The bottom edge of the rear metal plate is disposed in back of the linking slot. Also, the rear metal plate is relatively transverse to the axis defined by the symmetrical slot sections. Insomuch as the direction of the electromagnetic radiation in this embodiment is desired to be from the transition portion towards the antenna aperture, the shorted back plate acts to stop and absorb radiation in the opposite direction thereto; and U.S. Pat. No. 5,847,682 to Shyh-Yeong Ke entitled Top Loaded Tiangular Printed Antenna which describes a top loaded triangular printed antenna which will provide a planar antenna structure with broad bandwidth and high radiation efficiency. The antenna s structure has a vertical rectangular load, a triangular-shaped resonator having a smooth tapered section, a pair of grounded strips, a microstrip input transmission line, a grounding surface and a dielectric medium. Preferably, the grounded strips, the grounding surface and the rectangular load are metallic strip conductors printed on different planes of a dielectric medium of a printed circuit board.
An antenna can be generally considered as a special type of electrical circuit which is used in connection with a high frequency circuit. A transmission antenna efficiently transforms the power of a high frequency circuit into electromagnetic wave energy and radiates the electromagnetic wave energy in a space. A receiving antenna efficiently transforms the energy of input electromagnetic waves into power and transmits the power to an electrical circuit. As described above, the antenna serves as an energy transformer between the electrical circuit energy and electromagnetic wave energy, and its size and shape are appropriately designed to improve the efficiency of the transformation.
The bandwidth limitation of printed antennas is an inherent property, which comes from the resonant conditions at a single radiator. Thus, the bandwidth of a conventional patch radiator on a thin substrate is limited to 2% from its center frequency. The utilization of thick and multi-layer dielectrics provides a chance to increase the bandwidth by about 15% from its center frequency.
The use of a thick dielectric substrate can cause several problems. First, the excitation of surface waves is increased. Second, in the case of a printed feed network, the radiation losses are high. Third, the weight and cost of the device is increased. Fourth, there is a serious problem of reflection and radiation of a vertical feed. A very wide dipole was even shown to have a bandwidth of 37% from its center frequency (BAILEY. M. C. `Broadband half-wave dipole`, IEEE Trans., 1984. AP-32, pp. 410-412).
However, this antenna has the following disadvantages: a long distance between a grounded conductor plate and a radiator (about 0.39.lambda., where .lambda. is the wavelength); and a decrease in bore side radiation level (about 3 dB). These problems act as significant obstacles when the above antenna is used as a radiator consisting of an antenna array.